In order to meet future emission regulations in vehicles having a compression ignition engine, it may be necessary to use lean NOx catalysts in combination with particulate filters.
A NOx catalyst reduces NOx emissions continuously, even in an oxygen rich environment. For an active NOx catalyst to maximize NOx reduction, a reducing agent, for example, diesel fuel or urea, needs to be present. The optimum amount of reducing agent for the NOx catalyst is typically based on engine operating conditions and catalyst conditions. These conditions typically include engine speed, engine load, and catalyst temperature.
A particulate filter, also commonly used with compression ignition engines, is used to prevent soot, or carbon particles, from exiting the tailpipe. Since the particulate filter has a limited storage capacity, it is periodically regenerated. In one approach, during the regeneration process, exhaust temperature is increased to ignite carbon particles stored in the particulate filter. By burning the stored carbon particles, the filter is regenerated and able to again store the carbon particles. In addition, the burning of the carbon particles causes an increase in temperature.
One approach for managing a particulate filter estimates that amount of stored particulates and then regenerates the filter when this amount reaches a predetermined value. This decision can be augmented based on a vehicle operating zone defined by engine speed and load. Such a system is describe by EP 0 859 132.
The inventor herein has recognized numerous disadvantages when using prior particulate filter management systems when a NOx catalyst is present. In particular, the above management system does not consider the state of the NOx catalyst when determining filter regeneration. For example, the prior art does not consider conditions where heat generated from regeneration raise NOx catalyst temperature beyond appropriate limits.